The objective of a centrifugal compressor is to collect a steadily flowing stream of gas, pressurize the stream, and provide a steady pressurized stream to a subsequent process or device used in turbine engines and industrial processes. Instability can develop within the fluid being processed by a centrifugal compressor that interrupts the steady flow of fluid through the compressor. The instability can range in magnitude from weak to severe. Instabilities are commonly referred to as rotating stall and surge, or simply stall and surge conditions, where rotating stall is generally at the weak end of the scale and surge describes the severe condition. In a surging condition, flow direction through the centrifugal compressor can oscillate rapidly between the forward and reverse. Local oscillations of pressure disadvantageously occur within the compressor and adjacent components. Time-averaged supplies of pressure and flow to downstream processes are undesirably diminished. More particularly, catastrophic damage to the centrifugal compressor and adjacent components can result from pressure and flow oscillations. Further, downstream processes are disrupted with potentially serious consequences. In the case of a turbine engine, the production of thrust or shaft-horsepower is severely reduced or stopped altogether.
The flow-field instability leading to surge can develop in either the impeller or diffuser of the compressor, with diffuser initiated surge being the more severe case. There are various theories regarding the fundamental causes of surge that have been, shown to have merit through experimentation. The variety of substantiated theory suggests that many factors contribute to the development of surge. The factor that becomes the primary contributor to surge development varies with operating condition, compressor geometry and flow conditions at the inlet and discharge of the compressor. As such, it is difficult to prescribe a single solution.
There is little prior art to address the problem of controlling stall and surge in centrifugal compressors. Prior art for controlling these problems that does exist falls into five categories: 1) openings that are drilled between diffuser passages to permit communication between individual passages of the diffuser; 2) variable diffuser vane geometry or variable inlet guide vanes located upstream of the impeller that change matching between the impeller and diffuser or the operating characteristic of the stage; 3) devices that vary the pressure, temperature or flow conditions downstream of the compressor stage; 4) pulsed and steady injection of air in the general direction of diffuser flow at various locations on the diffuser vanes or diffuser passage endwalls; and 5) pulsed and steady injection of air into the impeller blades in the general direction of inlet flow from an upstream location.
A drilled opening between diffuser passages operates continuously over the entire operating characteristic of the compressor even though its contribution to stability is only required at one point. Hence, losses produced by this technique must be endured when stability enhancement is not required.
Variable geometry of diffusers is expensive to implement, as it requires numerous complex parts. Further, it disadvantageously activates slowly compared to the onset of stall and surge.
Downstream devices reduce pressure by bleeding process flow or by controlling a downstream device. The response of the compressor to a downstream control action is slow compared to control actions that act closer to the source of the instability. Bleeding process flow reduces process efficiency since work that has gone into pressurizing the flow is disadvantageously lost with the bleed-flow.
Injecting air into the diffuser or upstream of the impeller, related to categories 4 and 5, in the general direction of compressor through-flow has been shown to improve stability in certain cases when an external air source is used to supply injectors. The results are less significant when air from within the compressor flow path is used. Piping to deliver air at the injection points disadvantageously adds weight and complexity to an engine. As such, air injection techniques may be limited to ground based centrifugal compression systems, thereby reducing the possible commercial applications thereof. It is desired to provide devices and method of operation thereto for maintaining stable operation of centrifugal compressors by preventing stall and surge conditions therein and to do so without suffering the drawbacks of the prior art techniques.